1. Field of the Invention
The present invention relates to a method of manufacturing a flexographic printing plate having a base (polyester foil) and a soft caoutchouc mass which is enriched with photo-initiators (photo-polymer) and is arranged on the base, wherein a film with the image to be printed is fastened with precise fit on a free surface portion of the caoutchouc mass.
2. Description of the Related Art
Methods of the above-described type are basically known for the preparation of a flexographic printing method, which is a letter press printing method. In this method, printing ink is transferred to the printing plate. The printing plate transfers the ink under printing pressure in the printing machine to the material to be printed.
Available printing plate materials are vulcanized rubber plates, lasered rubber plates and lasered, rubber-coated cylinders, as well as photo-polymeric plates and cylinders coated with photo-polymeric layers.
Two systems of photo-polymeric printing plates are known on the market, i.e., the liquid system and the solid system.
In quality flexographic printing, predominantly solid plates are used whose market share is approximately 65% according to data provided by manufacturers.
An extremely soft caoutchouc mass enriched with photoinitiators is extruded in a thickness of 0.513 mm to 6.223 mm onto a polyester foil having a thickness of 0.127 mm to 0.229 mm and the surface of the caoutchouc mass is homogenized by means of calendars with total tolerances of .+-.10 .mu.m.
By adding the thicknesses of the base and of the polymer, printing plates having total heights of between 0.640 mm and 6.350 mm are produced.
For manufacturing the printing plate so as to be ready for printing, a right reading mat negative is required in which the portions to be printed later are permeable to light. The film and the photo-polymeric unfinished plate are exposed to ultraviolet light under vacuum in an ultraviolet light exposing device. By means of the penetrating ultraviolet light, the permeable portions in the film harden the portions of the plate which later transfer the ink. The non-exposed portions are washed out in a washer, so that the portions to be printed later are present on the plate laterally reversed and raised.
As a rule, the flexographic printing machines have six or eight printing mechanisms, so that it is possible to print with six or eight colors in one step of operation. All colors which are to be printed in one step of operation must fit together. In order to make the arrangement of colors relative to each other possible, concentrically arranged register marks are provided to the left and right of the printed image.
In this method, it is attempted to arrange all register marks of all six or eight color printing forms so as to essentially coincide. The register mark of the first color is positioned by means of mirror systems or by means of video cameras and the register marks of the subsequent plates are aligned in accordance with the first register mark.
Methods in which the holes are drilled or punched into the finished printing plate through hardened polymer and through the base also utilize register marks for positioning. Because of the high compressibility, also of the already hardened polymer, the resiliency of the plate itself makes it impossible to achieve exactly reproducible holes. In the case of plate thicknesses of 0.64 and 0.76 mm, a relief of 0.513 mm or 0.633 mm is present between the printing surface and the base. In all other printing plates, the relief depth between the printing surface and the polymer base are between 0.8 and 1.5 mm. Because printing portions alternate with non-printing portions, an area-parallel placement for the drilling or punching procedure is not possible.
The plate is resilient because of the different thicknesses. Moreover, polymer residues adhere to the tool and, as a result, increase the size of the respective hole in the base.
The registers of conventional printing methods, i.e., offset printing and gravure printing, are 0.050 to 0.100 m. In the present state of the art, an accurate assembly for offset printing is not possible in flexographic methods.
In the film manufacture of conventional methods, the reprotechnical registers are achieved by punching. In other words, the unexposed film is punched in a film puncher immediately after the removal from the film dispenser and is placed in an assembly rail with raised register pins and is possibly brought into contact with an already exposed, also punched film, and is then exposed to light. The accuracy to fit is not achieved by means of assembled register marks, but through the above-described punched hole system with accuracies of .+-.10 .mu.m.
The offset printing plate copying method operates in accordance with the same principle of punched holes between film and plate and the assembly through register pins.
The flexographic method operates in the manufacture of films also by means of film punching. However, the final film is already no longer punched because in the past it was not possible to further use punched final films with punched unfinished plates to be exposed to light.
The plate thicknesses which are used in flexographic methods vary between 0.644 and 6.35 mm.
The degrees of hardness between the unfinished plates and between the finished plates are also subject to extreme variations. In the unfinished plates, the degrees of hardness vary between 8.degree. Shore-A and 19.degree. Shore-A, and in the finished plates, i.e., after exposure to light and after treatment, the degrees of hardness are approximately between 42.degree. and 65.degree. Shore-A.
The soft unfinished plate makes it impossible to carry out punching or drilling with accurate fit. The punching and drilling procedure alone would damage the extremely soft polymer mass on the base when a tool is placed on the polymer mass and the printing plate would become useless as a result. During any drilling procedure as well as any punching procedure, the soft polymer would not permit an exact borehole within a mass or in the base. Polymer portions would be deposited on the outer side of the tool during the drilling or punching procedure and would pull a portion of this mass in drilling direction through to the base.
The material in the drilling location sags down. This makes an exact guidance of the register pin impossible because the holes in the base as well as the holes of the polymer are no longer round or accurate.